The present invention relates to an optical method of synthesizing arbitrary RF-lightwave or RF waveforms. Prior methods can generate multiple RF tones but have no provision for selectively adjusting the amplitudes of those tones. Known methods can be used to generate multi-tone RF combs amplitude-modulated on lightwave carriers. This invention improves upon such known techniques by filtering select lightwave frequencies and applying them to amplitude modulate the tones of the comb.
Prior art digital electronic synthesizers are quite versatile, but can produce waveforms that have bandwidths of only several hundred megahertz. Analog electronic synthesizers are capable of higher bandwidths, as high as several tens of gigahertz, but the waveforms are quite simple (comprised of only a few tones). The disclosed optical methods and apparatus of this invention, which allow for synthesizing the waveforms while in the lightwave domain, can produce waveforms with bandwidths in excess of one terahertz and that are comprised of a large number of tones.
The prior art includes the following:
(1). 1.8-THz bandwidth, tunable RF-comb generator with optical-wavelength reference—see the article by S. Bennett et al. Photonics Technol. Letters, Vol. 11, No. 5,pp. 551–553, 1999.                This paper describes multi-tone RF-lightwave comb generation using the concept of successive phase modulation of a laser optical waveform in an amplified circulating fiber loop. A phase modulator in an amplified re-circulating fiber loop generates the RF-lightwave frequency comb. In this comb generator, the lightwave signal from a laser injected into an optical loop undergoes phase modulation and optical amplification on each round trip. A series of optical sidebands spaced exactly by the RF modulation frequency applied to the phase modulator are generated.        
(2). Multi-tone operation of a single-loop optoelectronic oscillator—see an article by S. Yao and L. Maleki, IEEE J. Quantum Electronics, v.32,n.7,pp.1141–1149, 1996.                This document discloses a single loop optoelectronic oscillator. This oscillator contains a modulator, optical feedback loop, and photodetector. Although the intent of the authors is to generate a single tone by incorporating a narrow-band frequency filter in the loop, demonstration of multiple tones was achieved by enlarging the bandwidth of the filter. The frequency spacing of those multiple tones was set by injecting a sinusoidal electrical signal into the modulator, with the frequency of the injected signal equal to the spacing of the tones. This method causes all of the oscillator modes (one tone per mode) to oscillate in phase.        (3). Micro-ring resonators with absorption tuning for wavelength selective lightwave add/drop filtering—see the articles by S. T. Chu, B. E. Little, et al., IEEE Photonics Technol. Letters, Vol. 11,No. 6,pp. 691–693, 1999 and by B. E. Little, H. A. Haus, et al., IEEE Photonics Technol. Letters, Vol. 10,No. 6,pp. 816–818, 1998.        The Chu article provides experimental results verifying that a collection of micro-ring resonators can be used to separately filter a series of lightwave frequencies (or wavelengths). The second article provides an analysis that indicates the absorption, or loss, of the micro-ring resonator can be used to change the amount of light that is coupled into a micro-ring resonator and, thus, filtered.        
(4). Optical add/drop filters based on distributed feedback resonators—see the papers by R. F. Kazarinov, C. H. Henry and N. A. Olsson, IEEE J. Quantum Electron. Vol. QE-23,No. 9,pp. 1419–1425, 1987 and by H. A. Haus and Y. Lai, J. Lightwave Technol., Vol. 10,No. 1,pp. 57–61, 1992.                This paper provides the design for another type of optical filter that can have RF bandwidths. The design provides for a filter bandwidth of 10 GHz. Even smaller bandwidths could be realized using currently available fabrication techniques. The authors do not discuss how to change the amount of light that is filtered.        
(5). Optical add/drop filters based on Bragg gratings in interferometers—see the paper by F. Bilodeau, et al., IEEE Photonics Technol. Letters, Vol. 7,No. 4,pp. 388–390, 1995.                This paper describes the use of Bragg gratings in an optical-fiber interferometers configuration to accomplish the add/drop filtering. The authors do not discuss how to change the amount of light that is filtered. The filtering bandwidth of an optical Bragg grating is quite broad. A FWHM bandwidth of 25 GHz was reported for a Bragg grating of 1-cm length.        
The waveform synthesizer disclosed herein includes a RF-lightwave frequency-comb generator that is coupled to a multi-tone, frequency selective amplitude modulator. The continuous-wave (CW) comb is a set of RF tones that are amplitude modulated onto a lightwave carrier. The amplitudes of these RF tones can be given different weights by the frequency-selective modulator, and the values of these weights can be changed. Since a waveform is described by its Fourier spectrum, which is the amplitudes of its constituent frequency components, changing the values of these amplitudes will change the waveform that results. The generator of the RF-lightwave frequency comb is preferably a photonic oscillator or, alternatively, a single loop optoelectronic oscillator or a tunable re-circulating comb generator, the latter two of which are known per se in the art. The amplitude weights are applied preferably by a set of wavelength or frequency selective optical reflectors or couplers.
The present invention makes use of a single-tone RF reference to synthesize a variety of wideband RF-lightwave and RF waveforms. The RF lightwave waveform can be carried on optical fiber or transmitted through free-space optical links. The RF waveform is constructed by demodulating the complete RF waveform from a lightwave carrier using a photodetector. The highest frequency component of the synthesized waveform can have a frequency that is substantially higher than that of the RF reference.
Agile wideband waveforms are especially useful for optical communication systems with multiple users and for secure optical links. For example, each user can be assigned a particular and unique pattern for the amplitudes of the tones in the waveform. A user can then distinguish its signal from other signals that occupy the same band of frequencies by coherently processing the received signal with a copy of the particular waveform pattern of that user. This type of Code Division Multiple Access (CDMA) for lightwave waveforms is different from prior art techniques. Prior techniques make use of short optical pulses, much shorter than the information pulse, whose wavelength and sequence of temporal locations can be different for each user. The waveforms synthesized by the approach of this invention also could be used for wideband RF communications systems and links.
Agile, wideband waveforms can serve as carrier waveforms for low-probability of intercept (LPI) radar systems. The capability for amplitude weighing of the individual tones of the frequency spread multi-tone waveform provides a significant enhancement over the invention described in the patent application entitled “Agile Spread Waveform Generator” which is discussed above.